1. Field of the Invention
The present invention relates to a voltage measurement device, particularly to a threshold voltage measurement device.
2. Description of the Related Art
Variability is a critical problem in the systematic design of VLSI (Very Large Scale Integrated) circuits and likely to affect the threshold voltage of transistors. Threshold voltage correlates closely with performance, stability and reliability of electronic elements. Therefore, threshold voltage is an important index of variability and able to reflect the severity of the related phenomena and problems. Hence, it is necessary to create a circuit architecture for measuring threshold voltages of transistors, whereby data can be collected fast and massively to analyze threshold voltage variation and the influence of threshold voltage variation on the stability of chips.
Some prior arts use operational amplifiers to implement measurement of threshold voltages. Refer to FIG. 1 and FIG. 2. In FIG. 1, the output of an operational amplifier 10 connects with the gate of an N-channel FET (Field Effect Transistor) 12. The drain of the N-channel FET 12 connects with a high voltage. The source of the N-channel FET 12 connects with a low voltage via a resistor 14. The negative input of the operational amplifier 10 connects with the source of the N-channel FET 12. A preset voltage is supplied to the positive input of the operational amplifier 10. As the abovementioned connections implement a negative feedback circuit, the N-channel FET 12 generates a stable current. Thereby can be measured the threshold voltage of the N-channel FET 12. In FIG. 2, the output of an operational amplifier 16 connects with the gate of a P-channel FET 18. The drain of the P-channel FET 18 connects with a low voltage. The source of the P-channel FET 18 connects with a high voltage via a resistor 20. The negative input of the operational amplifier 16 connects with the source of the P-channel FET 18. A preset voltage is supplied to the positive input of the operational amplifier 16. As the abovementioned connections implement a negative feedback circuit, the P-channel FET 18 generates a stable current. Thereby can be measured the threshold voltage of the P-channel FET 18. Although the abovementioned measurement method can obtain the threshold voltage of a transistor, it does not apply to SRAM (Static Random Access Memory). Besides, the abovementioned method is economically inefficient because it needs expensive equipment and consumes much time but obtain only analog data.
In order to measure threshold voltage, some prior arts vary the physical structure of SRAM, including the polysilicon layer, the diffusion layer and the contact layer. Such a method would vary the physical characteristics and leakage current of SRAM, and causes SRAM to lose the advantages of the original physical structure.
Accordingly, the present invention proposes a threshold voltage measurement device to overcome the abovementioned problems.